The assailant, wearing an explosives belt under his clothes, apparently was allowed to enter the small base after offering to become an informant, according to two former agency officials briefed on the attack. The CIA declined to comment on the circumstances behind the incident, and it was unclear whether the bomber chose the base because of its role in supporting CIA airstrikes against top al-Qaeda and Taliban leaders in the region.

The blast early Wednesday evening in the eastern province of Khost killed seven CIA officers and contractors, including the base chief, and seriously wounded six others in what intelligence officials described as a devastating blow to one of the agency's key intelligence hubs for counterterrorism operations. It was the deadliest single day for the agency since eight CIA officers were killed in the 1983 bombing of the U.S. Embassy in Beirut.

The CIA continued drone strikes Thursday. A security official in Pakistan confirmed that two militants were killed late in the day in what was described as a missile attack by a Predator drone in Pakistan's autonomous North Waziristan region, across the border from Khost.

The official said the missile destroyed the home of a man believed to be linked to the extremist group Tehrik-e-Taliban. The CIA has consistently declined to acknowledge any participation in the ongoing campaign of airstrikes that killed more than 300 people in the past 12 months.

U.S. intelligence officials vowed that the Wednesday attack would only increase the agency's resolve. "This attack will be avenged through successful, aggressive counterterrorism operations," said one official, speaking on the condition of anonymity.

The CIA deaths were formally acknowledged by the agency in a statement to employees Thursday by Director Leon E. Panetta, who said the heavy toll was a reminder of the "real danger" that confronts intelligence officers on the fronts of the two wars. CIA operatives in Afghanistan volunteer for the posting and spend a year or more on assignment. Many of the slain -- including the base chief, a mother of three young children -- were seasoned hands in the agency's counterterrorism operations.

"Those who fell yesterday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism," Panetta said in his message to employees. "We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives -- a safer America." Panetta said military doctors and nurses had saved the lives of gravely wounded officers, and he announced that flags at CIA headquarters in Langley would be flown at half-staff to honor the dead.

As is customary, the CIA declined to identify the victims. Director of National Intelligence Dennis C. Blair did not publicly comment on the deaths, but a spokesman said he sent an internal, classified message expressing his condolences.

President Obama posted a letter to CIA employees honoring those killed, whom he called "part of a long line of patriots who have made great sacrifices for their fellow citizens, and for our way of life."

'Sloppy' screening

U.S. personnel at the site of the attack, Forward Operating Base Chapman, are heavily involved in the selection of al-Qaeda and Taliban targets for drone aircraft strikes, according to two former intelligence officials who have visited the facility. The drones themselves are flown from separate bases in Afghanistan and Pakistan. Because of its location near a hotbed of insurgent activity, the base is also a center for recruiting and debriefing informants, the officials said, and it would not be unusual for local Afghans to be admitted to the facility for questioning.

"There's still a lot to be learned about what happened. All the facts are not in," CIA spokesman George Little said. "The key lesson is that counterterrorism work is dangerous."

A Taliban spokesman asserted responsibility Thursday for the bombing and said the bomber was an Afghan National Army officer who had joined insurgents in attacking the United States. That description could not be confirmed with U.S. military officials. But a U.S. military official in Afghanistan, speaking on the condition of anonymity, said Afghan forces are posted at the base.

Forward operating bases in Afghanistan depend on locals for security. But insurgents have frequently infiltrated the ranks of Afghan security forces as well as private firms hired to guard U.S. facilities or to perform more menial tasks. CIA officials on Thursday would not discuss what guard service they had at the base.

Thomas Ruttig, co-director of the Afghanistan Analysts Network, said that this week's attack once again shows that there "needs to be much better screening of people joining the Afghan security forces. . . . I know from visits in Afghan provinces this is done in a very sloppy way."

The danger of infiltration, he added, could increase as the U.S. military seeks to develop "community defense forces."

Severed communication

Forward Operating Base Chapman is a former Afghan army installation and was used jointly by American and Afghan security forces during their military campaign against the Taliban beginning in 2001. In recent years, the base added an intelligence-gathering function and had a housing compound for U.S. intelligence officers. It was physically separate from the main U.S. military base nearby, Forward Operating Base Salerno.

Senior Afghan civilian officials in Khost said that they knew little about what went on at Chapman and that since Wednesday's attack, they have been unable to reach anyone inside by phone. Afghan interpreters working on the base at the time have since been incommunicado, and those who were on leave that day have not been allowed back inside, according to Khost residents and officials reached by phone.

A spokesman for the Afghan National Army in Kabul denied that the Khost attack was carried out by a member of the army, but the possibility highlights growing concerns in Afghanistan and Pakistan about whether it is possible to sustain the loyalty and unity of their respective armies. The Afghan army, a crucial element in the new U.S. strategy in Afghanistan, is young, untested and ethnically diverse. It is being asked to fight fellow Muslims from the dominant Afghan tribe in an unpopular war on behalf of American forces and policies that many Afghans deeply resent.

"This attack shows that the Taliban are getting good cooperation from the locals and that they have better intelligence than the Americans do," said Talat Masood, a Pakistani security analyst and retired general in Islamabad, Pakistan's capital. "It also raises the issue that has haunted the Afghan National Army from the beginning -- whether or not it is possible to build a unified army that can overcome ethnic loyalties in support of broader American goals."